This invention relates to the manufacture of polyolefin resins, and in particular to the manufacture in a single reactor of polyolefin resins having polymodal, especially bimodal, molecular weight, density, or other characteristics. For bimodal effects, it employs two separate bimetallic catalyst compositions fed in ratios which may be varied to control the bimodal properties of the product more closely than has been possible in the past. Additional catalyst species may be employed in more complex processes.
The term xe2x80x9cbimodalxe2x80x9d as applied to polyolefin resins usually means that the resin has two distinct ranges of molecular weight or density, which can impart desired properties to the product in great variety. Originally, bimodal resins were made in two separate reactors or reaction chambersxe2x80x94that is, a product having a first molecular weight was moved directly from the reaction zone in which it was made and introduced to a reaction zone having conditions for making a resin of a different molecular weight, where more resin was made. The two resins are thus mixed or, in some cases, even present in the same particles. Various 2-stage and bimodal processes are reviewed by Cozewith et al in U.S. Pat. No. 4,786,697. Two-stage processes are difficult to control and, perhaps more important, have a capital disadvantage in that two reactors, or at least two reaction zones, are required to make them. Moreover, frequently the products are not homogeneously mixed, in that at least some particles will be entirely of one mode or the other. It is therefore desirable to find ways of making homogeneous bimodal polyolefins in a single reactor.
One approach to making bimodal polyolefins in a single reactor has been to employ a mixed catalyst system, in which one catalyst component (because of specific termination and/or chain transfer kinetics) makes a primarily low molecular weight (LMW) product, and the other catalyst component produces a primarily high molecular weight (HMW) product, because of different termination and/or chain transfer kinetics. By including both of these catalyst components in the same catalyst composition, a bimodal product may be produced; the molecular weight modes of the product will be intimately mixed, providing a resin product that is relatively free of gels compared to similar products made in staged-reactor processes or by the blending of two distinct unimodal resins.
In addition to tailoring the molecular weight distribution of the polymer, the different comonomer incorporation kinetics of specific catalysts can be applied in making products that are bimodal in density. A catalyst with favorable kinetics can incorporate alpha-olefins into polyethylene very effectively. A mixed catalyst system that uses two catalysts having different comonomer incorporation efficiencies can be used to produce such bimodal density products. Producing bimodal products in a single reactor relieves the necessity of a separate blending step, and allows them to be produced more quickly and efficiently.
Controlling the ratio of the components in the bimodal product is a significant manufacturing concern. Product properties of bimodal resins are often extremely sensitive to component split. For instance, in the manufacture of high-density, high-molecular-weight film, to achieve the desired specification requires control of component split within xcx9c2% of the setpoint.
The weight percentage, or xe2x80x9csplit,xe2x80x9d of HMW or high density (xe2x80x9cHDxe2x80x9d) in the total product including low density (xe2x80x9cLDxe2x80x9d) components in a single-reactor manufactured bimodal resin is primarily a function of the relative amount of each type of catalyst in the catalyst system. While, theoretically, a catalyst system containing proper amounts of each catalyst could be generated and used to produce the desired split in a particular case, in practice using such a system would be difficult, as the relative productivities of the catalyst components may change with variations in reactor conditions or poison levels.
There have been attempts in the past to control product component split by controlling catalyst split. Disclosed in WO 96/07478 is a method for determining the molecular weight distribution of a bimodal product made in a single reactor, which uses a supported bimetallic catalyst system with the addition of a make-up feed consisting of one of the metallic components. While this scheme can be used to control split, it has a major disadvantage in that the resin produced may contain particles consisting of only one component. The resulting heterogeneity of the resin is known to degrade product film appearance and performance.
Another proposed method for controlling product component split is the use of separate feeds for the HMW and LMW components. This method is most practical in liquid catalyst systems, where the components will become intimately mixed before polymerization begins. This method has a major disadvantage in that the catalyst split is very sensitive to fluctuations in the relative feed rates of each catalyst.
Each of the above described approaches to the problem of making bimodal polyolefins in a single reactor has difficulties and shortcomings. The art is in need of a method of making bimodal products with improved control and convenience.
One of the most challenging aspects of bimodal (in molecular weight distribution or density) polyolefin production in one reactor is the control of component split. This invention includes the simultaneous use of two multi-component catalyst blends to achieve a desired split control. Preferably, the first blend contains the same catalytic species as the second blend, but in different relative amounts. By simultaneously feeding the two catalyst blends to the reactor and varying the relative feed rates, the resin component split can be controlled at the desired setpoint.
In our invention, we feed two complex catalyst compositions or mixtures (blends), each capable of making both HMW and LMW components, or HD and LD components, or bimodal in some other aspect, such as productivities or reaction rates with respect to a comonomer, but (where the components are the same) having different fixed ratios of catalysts. By controlling the ratio of the two multi-component fixed-ratio catalyst blends, we can modulate or otherwise control the ratio of HMW product to LMW product (or other bimodal feature) rather precisely within a desired range.
By using only mixed catalyst compositions having fixed ratios of catalyst species, we avoid the possible manufacture of particles of only high or low molecular weight. Each particle will be a product of the mixed system. Further, the system is substantially less sensitive to perturbations in catalyst feed rates or feed ratios.
By feeding two distinct catalyst compositions, each having LMW and HMW producing components, the possibility of making particles of only high or only low molecular weight is avoidedxe2x80x94all resin particles in this invention will contain both HMW and LMW components. Furthermore, carefully choosing the composition of each catalyst mixed composition will ensure that each resin particle has a HMW-to-LMW ratio that lies in a range known or believed to produce acceptable film or other properties. Our method is effective in controlling the product split (the weight percent of HMW component in the overall product) with supported catalyst systems, spray-dried catalyst systems, or liquid phase catalyst systems.
The wide variety of specific catalysts we can use is illustrated in the following review of catalyst compositions useful in olefin polymerization.
Bimetallic catalysts are described by Kissin et al in U.S. Pat. No. 6,001,766. At least one of the two transition metal compounds they use is a cyclopentadienyl compound, and the resulting catalyst composition is said to produce polymer of broad molecular weight distribution. The ratio of the cyclopentadienyl compound (which preferably includes zirconium) to the other transition metal may vary.
Various other patents owned by Mobil Oil Corporation, such as Mink et al U.S. Pat. No. 5,614,456, Nowlin et al U.S. Pat. No. 5,539076, Mink et al U.S. Pat. No. 5,525,678, and Mink et al U.S. Pat. No. 5,882,750 describe catalysts said to be useful to make resins having bimodal characteristics. Mink et al in ""678 discuss the relative productivities of two metal catalyst sites. Blends of low and high molecular weight resin are said to be made by a titanium-zirconium bimetallic catalyst; various densities and molecular weight distributions are achievable using different conditions and combinations of the catalysts. The Mink et al ""750 patent purports to control the high molecular weight fraction of a bimodal product over a wide range, using a metallocene transition metal component and a non-metallocene transition metal component.
Ewen et al, in U.S. Pat. No. 4,937,299, utilize both components in the form of metallocenes having different reactivities; they produce a homopolymer and a copolymer simultaneously. In U.S. Pat. No. 5,242,876, Shamshoum uses a combination of a metallocene and a conventional Ziegler-Natta catalyst to obtain a blend of polymers with different desired properties. Samuels et al U.S. Pat. No. 4,918,038 use combinations of vanadium and/or vanadium oxide or zirconium species. Bergmeister et al in U.S. Pat. Nos. 5,648,439 and 5,624,877 describe a system of two chromium catalysts to make multimodal resin products. Benham et al in U.S. Pat. No. 5,237,025 utilize a chromium and a titanium catalyst to make bimodal (col. 8, line 66) products; however, the two catalytic sites are physically separated. See also Stricklin U.S. Pat. No. 4,939,217, utilizing two different metallocenes with different termination rate constants; they are not used in the same catalyst composition so as to eliminate the possibility of particles without bimodal distribution. A vanadium/zirconium system is used by Samuels et al in U.S. Pat. No. 4,918,038 to obtain a desired molecular weight distribution. Bimetallic metallocenes are used by Davis in U.S. Pat. No. 5,442,020.
A family of mixed metal catalysts is described by Cann et al in U.S. Pat. No. 5,442,018. Although they are described for use in tandem reactors for making bimodal resins, they can be used in our process as well; as described elsewhere herein, the preferred method would be to utilize two of the biselective catalysts, having different ratios of the same catalyst components. An example of a bimodal catalyst composition useful in our invention as described by Cann et al in U.S. Pat. No. 5,442,018 is a mixed metal catalyst comprising a titanium complex that is the reaction product of a titanium-containing compound in which the titanium is in the +3 or +4 oxidation state, a magnesium halide and a first electron donor, and a vanadium complex that is a vanadium-containing compound in which the vanadium is in the +2, +3 +4, or +5 oxidation state optionally reacted with a second electron donor, optionally used with the modifier(s) and the cocatalyst described therein (column 3, line 49-68).
We may use any of the polyselective catalyst compositions described in the above mentioned patents. The patent numbers are hereby repeated, as their entire specifications are incorporated herein by reference: U.S. Pat. Nos. 6,001,766, 5,614,456, 5,539,076, 5,525,678, 5,882,750, 4,937,299, 5,242,876, 5,648,439, 5,624,877, 5,237,025, 4,918,038, 4,939,217, 5,442,018 and 5,442,020. All of these patents disclose the use of a multi-species catalyst systems, most of them referring to the catalysts as bimetallic, but we will call them biselective or polyselective, in that the metals or species are chosen for particular selected properties or functions. In our invention, they may be chosen for producing different molecular weights under the same conditions, or producing resins of different densities, or for having different productivities, perhaps with respect to comonomers, for differing susceptibilities to hydrogen termination, or for other features or properties. While we may use catalysts having three or more such differing functionalities (triselective or polyselective), we prefer to have only two catalyst species in a given catalyst composition.
Our use of polyselective catalysts is not limited to the catalysts described in the above enumerated patents, which does not represent an exhaustive list of such known olefin polymerization catalysts. As our invention comprises a technique for controlling the composite product of a catalyst system, we utilize two or more polyselective catalysts. However, it is preferred to use only two polyselective catalysts, and it is preferred that they will each have the same metal or other active species but in different ratios. For our purposes, a biselective catalyst is one which has two different types of polymerization catalyst species in the same catalyst composition; a polyselective catalyst is one which has two or more different types of polymerization species in the same catalyst composition. Most often, this means that two species are present on the same support. Less frequently, the support itself will act as one of our active catalyst species, and will support a different catalyst species. In either case, since the two species are present in the same composition, and will polymerize the olefin(s) simultaneously, there is little or no chance that resin particles will be made including only one mode of resin product.
While for most purposes we prefer to feed two different solid catalyst compositions, each having at least two metal catalyst components in fixed ratios, it is also possible within our invention to feed a solid bi- or polyselective catalyst composition simultaneously with a liquid bi- or polyselective catalyst composition and/or two bi- or polyselective liquid compositions so long as the liquids are mixed in fixed ratios prior to entering the reactor.
For controlling bimodal molecular weights, our invention preferably employs two mixed (biselective) catalyst compositions, thus overcoming the problem of sensitivity to fluctuations in the relative feed rates for separate HMW and LMW catalysts. For instance, if one biselective catalyst blend independently generates a product with a 70% HMW, 30% LMW split and the other generates a 50% HMW, 50% LMW product, the range of products possible for all relative catalyst feed rates would be from 50 to 70% HMW, compared to a range of 0 to 100% if separate HMW and LMW producing feeds are used. This restriction in the range of possible products significantly reduces the sensitivity of the overall system to perturbations in relative catalyst feed flow rates.
It should be observed that our invention does not include the use of a single polyselective catalyst or a single polyselective catalyst fed simultaneously with a monoselective catalyst. Our invention requires the use of at least two polyselective catalyst compositions. Because we use two or more polyselective catalyst compositions, we are able more precisely to control and/or vary the ratio of product made by one catalyst species to product made by another catalyst species.
Generally, our invention includes a method of controlling the polymodal split of a property of a resin product of an olefin polymerization process comprising conducting the polymerization process in the presence of at least two polyselective catalyst compositions, and controlling the ratio of the polyselective catalyst compositions to each other during the polymerization process to achieve a desired polymodal split of the subject property. The polymodal split may be a bimodal split, in which case the polyselective catalyst compositions are biselective catalyst compositions.
More particularly, our invention includes a method of controlling the bimodal characteristics of polyolefin resin comprising making the polyolefin resin by polymerizing one or more olefins in the presence of two polymerization catalyst compositions A and B, each of the two polymerization catalyst compositions A and B including selected ratios of catalyst species X and catalyst species Y for making polymer molecules of selected characteristics of desired values, and controlling the ratio of the catalyst compositions A and B to each other during the polymerization to obtain a resin product having a desired bimodal split. As an example, catalyst species X may comprise bis(n-butylcyclopentadiene) zirconium dichloride and catalyst species Y may comprise titanium (Ti+3 and Ti+4) zirconium, Vanadium (V+4 and V+5), or hafnium.
The feed rates of catalyst compositions A and B may be manipulated in response to continuous or intermittent measurements, or a process model, of the desired product property or properties. The ratio of catalyst species X to catalyst species Y in a given biselective catalyst composition may be selected to provide a specific ratio of product having the property, or value thereof, of interest under a known set of polymerization conditions. The catalyst composition may then be referred to as one which provides a predetermined content, or xe2x80x9csplit,xe2x80x9d of, for example, high molecular weights compared to the overall product, which may differ from the weight or molar ratio of the metal components of the catalyst composition.
In principle, any two biselective or other polyselective catalyst compositions may be used in our invention, so long as they have an acceptable degree of effectiveness in imparting the property or properties desired. Typically they will be bimetallic or polymetallic, but they may be biselective or polyselective for reasons other than the type of metal polymerization site. For example, the catalyst components may respond to different promoters or modifiers, and/or they may respond to chain terminators such as hydrogen in different ways or in different degrees. Our invention utilizes the mathematical advantages of the manipulation of two different pairs (or other plurality) of catalyst sites as explained with respect to the equations discussed hereafter.